Geologic formations defining a reservoir for the accumulation of hydrocarbons in the subsurface of the earth contain a network of interconnected paths in which fluids are disposed that ingress or egress from the reservoir. To determine the nature and behavior of the fluids in the aforementioned network, knowledge of both the nature of the pore fluids and the porosity of the geologic formations is desired. With this information, efficient development and management of hydrocarbon reservoirs may be achieved.
For example, the electrical resistivity of geologic formations is a function of both porosity of the formations and resistivity of the fluids. Considering that hydrocarbons are electrically insulating and most formation water is saline and thereby electrically conductive, resistivity measurements provide valuable data to determine the presence of hydrocarbon reservoirs in geologic formations. Based on resistivity measurements it is further possible to monitor the changes in hydrocarbon content as production of the hydrocarbon proceeds and water saturation increases.
In the prior art, methods and tools have been described and used to determine the electrical resistivity of geologic formations surrounding and between boreholes. In the context of the present invention, tools and methods sensitive to inter-well formation structures are referred to as “deep reading” to indicate a monitoring of resistivity in formations away from the immediate surroundings of a single borehole. Deep-reading electromagnetic field surveys of subsurface areas typically involve large scale measurements from the surface, from surface-to-borehole, and/or between boreholes. Deep reading tools and methods are designed to measures responses of the reservoir on a scale equivalent to a few percent of the distances between boreholes. This is in contrast to the established logging methods, which are confined to the immediate vicinity of the boreholes, i.e. typically within a radial distance of one meter or less. Deep reading methods are applied for determining parameters of the formation at a distance of 10 meters or more up to hundreds of meters from the location of the sensors. Field electromagnetic data sense the reservoir and surrounding media in this large scale sense.
Details on deep reading methods and tools for inter-well formations can be found for example in the two articles, “Crosshole electromagnetic tomography: A new technology for oil field characterization”, The Leading Edge, March 1995, by Wilt et al. and “Crosshole electromagnetic tomography: System design considerations and field results”, Geophysics, Vol. 60, No. 3, 1995 by Wilt et al. In both sources, the measurement of geologic formation resistivity is described employing low frequency electromagnetic (EM) systems. More recent deep reading surveys are described in “Crosswell Electromagnetic Tomography in Haradh Field: Modeling to Measurements”, SPE 110528, Society of Petroleum Engineers, by Marsala et al. and in “Crosswell Electromagnetic Tomography in Saudi Arabia: from Field Surveys to Resistivity Mapping”, presented at the EAGE conference, 9-12 Jun. 2008, by Wilt et al.
Methods and tools for performing EM measurements further are described in a number of patents and patent applications including U.S. Pat. No. 6,393,363 to Wilt and Nichols and other patents and patent applications.
In view of the known art, it is seen as one object of the invention to improve and enhance the effectiveness of deep-reading electromagnetic surveys. It is seen as a particular object of the invention to accelerate the evaluation of a survey, thus make deep-reading a potential tool for in-situ or quasi in-situ control of field development operations such as well drilling, water flooding or enhanced oil recovery (EOR).